This invention relates generally to an improvement in linear actuators and more particularly, but not by way of limitation, to a failure tolerant linear drive mechanism particularly adapted for celestial space applications.
The present invention is directed to a failure tolerant, direct drive, linear motion mechanism which inocludes first and second high torque motors and an associated output shaft. The mechanism is adapted to convert rotational motion into longitudinal non-rotating reciprocative shaft motion when the motors are operated singly or concurrently. The motor to shaft coupling of each motor provides for operation of either motor without disengaging the other. In addition, the coupling means enables unimpeded operation of the linear drive mechanism in the event of a motor failure such as freewheeling or seizure. The present invention is intended for applications where continued use of the linear drive mechanism is essential if not critical. It will be appreciated that in aerospace applications and especially celestial space applications a linear drive mechanism must be able to operate at all times and tolerate failure of an actuating motor to prevent failure of the mission, loss of life or loss of property. Heretofore, the prior art has not disclosed a self contained failure tolerant linear drive mechanism which can tolerate a motor seizure and yet continue to operate.
U.S. Pat. No. 2,520,014 issued to Axel F. Rehnberg et al., discloses an industrial electrical control mechanism for machine tools which utilizes two remotely located electric motors which are separately located and separately mounted. The electric motors are connected to a feed screw by means of spline and screw shaft couplings along with associated belts, pulleys, and reduction gears. The mechanism is intended for use as an industrial drive mechanism and is not a failure tolerant integral linear actuator particularly adapted for space vehicle use.
U.S. Pat. No. 3,803,927 issued to Martin D. Lawler relate to a coaxial force and movement mechanism for a machine tool. The Lawler mechanism, like the Rehnberg mechanism, discloses a mechanism that is intended as part of an industrial control. It shows a remotely mounted and separately housed motor that is coupled to a shaft through a spline coupling and which, in association with a remotely located drive motor, causes linear axial movement of the shaft and a quill relative to a frame. Lawler does not disclose a self contained integral linear drive mechanism that is failure tolerant and which is particularly adapted to space vehicle applications.
The Schrader U.S. Pat. No. 2,860,266 and the Westmoreland U.S. Pat. No. 3,407,680 are similar in that they show self contained linear actuators which rely upon concurrent running and differential rotational speeds of two electric motors to provide reciprocative movement of a shaft. However, both mechanisms are not failure tolerant and if one motor fails the other motor will turn the rotor of the failed motor at the same speed and direction of the still running motor and since the resistance of the non-running motor to the rotational influence will normally be less than the resistance required for linear movement of the output shaft (due to externally imposed axial loads), the force produced by the running motor can be spent turning the rotor of the non-running motor thus producing no linear movement. Neither patent discloses a failure tolerant linear actuator that will still produce linear movement even if one of its motors were to fail.
The Thomas O. Paine U.S. Pat. No. 3,660,704 discloses a ball screw linear actuator that is particularly suited for use aboard the space craft. It discloses a ball screw nut assembly and motor that are concentrically arranged with respect to an actuation shaft but it does not show any tolerance for failure of the drive motor and is exemplary of the prior art and its associated problems that the present invention shows an advance over.